In cigarette manufacturing machines, shredded tobacco is normally fed via an input hopper to a gravity channel connected via a carding unit to a basin, from which extends upwards an upflow channel closed at the top end by a conveyor belt permeable to air. In the upflow channel, an upward air current, at least partly produced by suction through the conveyor belt, draws up the light part of the tobacco comprising powder and relatively minute shreds, while any heavier parts, such as lumps, woody parts, or foreign bodies (stones and similar) fall by gravity into the basin and are rejected.
Though widely used and relatively effective, the above method may result in problems caused by the heavier parts, particularly the foreign bodies, damaging the carding unit.